The invention relates to an apparatus and to a method for determining the viscosity of a fluid.
Newton""s law of friction describes the intensity of the internal friction between adjacent layers in a flowing fluid for the simple case of a laminar flow as, for example, in a flowing liquid or in a gas. The internal friction force FR, which acts on such a layer in the flowing fluid, is proportional to its area A and to a velocity gradient dv/dx (v: velocity of the flowing fluid at the location x) with respect to an adjacent layer of the same area. In the case of a laminar flow, the well-known simple relationship applies of FR=xcex7xc2x7A xc2x7dv/dx, where the dynamic viscosity xcex7 is to be seen as a measure for the viscosity of the fluid. In addition to the correct SI unit Paxc2x7s for the dynamic viscosity, the unit 1 poise=0.1 Paxc2x7s is still very common. In addition to the dynamic viscosity xcex7, the so-called kinematic viscosity xcexd=xcex7/xcfx81(xcfx81: density of the fluid) normed to the density of the fluid represents an important characteristic of a fluid. Within the framework of this application, viscosity is always to be understood as the xe2x80x9cdynamic viscosity xcex7xe2x80x9d following usual language use.
As the above statements show, the viscosity can generally be determined by a measurement of the internal friction FR of the fluid. For this purpose, a sample of known geometry is moved in a static or flowing fluid, with a measurable proportion of the driving power of the sample being converted into heat as power loss due to the internal friction FR in the fluid. Ultimately, the viscosity xcex7 and/or the kinematic viscosity xcexd can be determined from the measured power loss, possibly while taking into account further parameters such as geometric factors or further calibration parameters.
In practice, a sample of known geometry is rotated in a static or flowing fluid by means of an electrical drive and the viscosity xcex7 is determined from the measured electrical power loss of the drive. The specific shape of the sample can be taken into account, for example, by calibration measurements on fluids of well-known viscosity. It must furthermore be taken into account that the viscosity is also dependent on physical state quantities such as on the temperature or on the pressure of the fluid and is generally to be considered as a function of further physical characteristics such as the density.
Viscosimeters, which work while taking into account the above constraints and in accordance with the previously described principles or according to related principles, have long been known in a number of different variants. In order to be able to determine the viscosity of the fluid from the electrical power taken up by the rotary drive of the viscosimeter, that proportion of the total power loss must be known as accurately as possible which is due solely to the internal friction in the liquid. This means in particular that all friction losses which occur for example in bearings of the rotary drive themselves or at seals of feedthroughs of drive rods, etc., must be known very precisely. Particularly these parameters are, however, as a rule not sufficiently known in conventional electrical drive systems for viscosimeters. Furthermore, the previously mentioned, unwanted additional friction losses can depend on the current operating parameters under which the viscosimeter has to be operated, such as on the temperature or on the density of the fluid to be investigated and even on the actual viscosity to be investigated (low or high viscosity, gas or liquid). For this reason, the measuring precision of known viscosimeters is frequently unsatisfactory, or a substantial additional effort must be made to calibrate the measuring arrangement, with the calibration having to be made again each time for a change in the operating parameters and/or the conversion of the measurement to a fluid with different physical characteristics. This circumstance in particular impedes or hinders a precise determination of the viscosity when this changes in the course of the measurement, i.e. when the viscosity of a process is to be determined xe2x80x9cin-linexe2x80x9d. For instance, the time development of slow chemical reactions (in the liquid or gaseous milieu) in a reaction vessel, for example, can very frequently only be observed with insufficient precision by a time-dependent measurement of the viscosity with the known viscosimeters. The viscosity within an inhomogeneous fluid can also depend greatly on the place. For instance, the mixing of the fluid in a reaction vessel could, for example, basically be controlled via the viscosity, which is, however, only possible with the known viscosimeters with a great effort and/or the tolerating of massive measuring errors.
Special problems can occur if the viscosity of chemically or physically aggressive fluids is to be determined. For instance, with special liquids, which contain solid particles for example, substantial damage can occur at sealing components due to abrasion, for example at feedthrough seals of the drive rods. The same applies to chemically aggressive liquids such as acids or lyes which can permanently damage the sealing rings of drive components.
In practice, numerous processes and methods can be found in which the constant monitoring of the viscosity xcex7 can play an important role. For example, a layer of photo-resist of a well-defined thickness has to be applied to a wafer in the production of integrated circuits. Usually, for this purpose, the wafer is set into fast rotation with a defined speed and the photo-resist is applied to the rotating wafer via a feed device. In this connection, the larger part of the applied photo-resist is slung off due to the rotation of the wafer. The resulting layer thickness on the wafer is substantially greatly dependent, among other things, on the viscosity xcex7 of the photo-resist to be applied. It would therefore be of advantage to continuously control the viscosity xcex7 of the photo-resist before application and possibly to correct it by mixing in one or more additional components; i.e. the viscosity xcex7 should be monitored in-line and with high precision. A further important example to be named is that of chemical-physical polishing processes, so-called CMP processes (chemical-mechanical polishing), which likewise enjoy wide application in the semi-conductor industry. In such processes, a suspension known as a slurry is usually used as the polishing liquid which contains very fine solid particles whose uniform distribution in the suspension can be monitored especially simply by a measurement of the viscosity xcex7.
It is therefore an object of the invention to provide another apparatus for measuring the viscosity of a fluid by means of a rotating sample which allows the most accurate determination possible of the viscosity, in particular also in-line, that is, in the course of the process. Furthermore, a method should be provided for operating such an apparatus.
In accordance with the invention, an apparatus and a method are provided for determining the viscosity of a fluid, with the apparatus including an electrical rotary drive having a stator with a stator winding and with a rotational body rotatable in the fluid. The rotational body is designed as a rotor of the rotary drive and magnetically journalled in a contact-free manner with respect to the stator.
It is important for the invention that the drive of the rotational body is not connected with any friction losses in the form of bearing friction or similar. This is achieved in that the rotational body, which is designed as a rotor of the electrical rotary drive of the apparatus in accordance with the invention, is magnetically journalled in a contact-free manner with respect to the stator. The rotor is thus mechanically completely uncoupled from the other components of the rotary drive. The electrical driving power to be applied for the rotation of the sample in the fluid thus substantially depends only on the viscosity of the fluid and is not also determined by additional losses. If no hydraulic pump performance at all is provided by the rotational body in operation, the torque forming current of the rotary drive in static operation is determinedxe2x80x94with the exception of negligible ohmic lossxe2x80x94only by the internal friction of the fluid in which the rotational body rotates, that is, solely by the viscosity of the fluid.
The electrical rotary drive for the apparatus in accordance with the invention for determining the viscosity of a fluid is preferably designed in accordance with EP 0 819 330 or EP 1 063 753 as a bearing-free motor with a magnetically journalled rotor and a stator, with the drive and the magnetic support for the rotor forming a unit in accordance with the principle of a bearing-free motor. The stator has a drive winding with a strand for generating a magnetic drive field which produces a torque on the rotor. The magnetic drive field is in this connection controlled by a torque forming current. In addition, the stator has a control winding including at least one strand for generating a magnetic control field with which the position of the rotor with respect to the stator can be regulated via a control current. The magnetic drive field and the magnetic control field can be completely uncoupled from one another so that the torque forming current of the drive winding feeds the total mechanical motor power on its own. The total mechanical motor power which has to be applied on the operation of the bearing-free motor as the rotary drive of the apparatus in accordance with the invention results as the sum of viscous power loss caused by the rotation of the rotational body in a fluid and a possibly simultaneously supplied pump power by pumping the fluid against a pressure difference. In this connection, the pump power results simply by multiplication of the pumped flow by the corresponding pressure difference, while the mechanical motor power is proportional to the product of torque forming current and rotational frequency of the rotational body in the fluid. With a known hydraulic pump power, the viscous power loss can thus be determined directly from the torque forming current and the rotational frequency of the rotational body and from this the viscosity xcex7 of the fluid. In particular with a very small or negligible hydraulic pump power, the viscosity xcex7 of the fluid is thus directly proportional to the torque forming current with a given constant rotational frequency.